1. Field of the Invention
The present invention relates generally to an apparatus and a method of searching for a PN (Pseudo-random Noise) sequence phase in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to an apparatus and a method of searching for a PN sequence phase in a CDMA mobile communication system using a multi-carrier.
2. Description of the Related Art
The CDMA mobile communication system has been developed from transmission/reception of voice signal to IMT-2000 which can afford transmission of data at high speed as well as voice transmission. The objectives of IMT-2000 are the transmission of high quality voice data and moving pictures, and high speed internet browsing, etc. A multi-carrier scheme has been suggested for an IMT-2000 system in which intended information is transmitted over a plurality of carriers. The multi-carrier scheme is a process of modulating information spread by the same PN sequence with different carriers prior to transmission.
A transmitter in a multi-carrier CDMA system converts an information signal to a plurality of parallel signals and multiplies the parallel signals by a PN sequence, for spreading. Then, the transmitter multiplies each of the spread signals by different local carriers for modulation. Thus, data is transmitted in different bands. When the transmitter converts the information signal to parallel signals, the information signal is separated into a plurality of band signals in a base band and converted to an RF signal by multiplying the band signals by a predetermined single carrier corresponding to a central frequency. A receiver demodulates the information signal by multiplying each band signal by a corresponding local carrier.
A base station (BS) in the multi-carrier CDMA system transmits pilot signal modulated by PN sequence with its own PN phase offset. The MSs search for the respective PN sequence phases of the multi-carrier signals in a serial search scheme or a parallel search scheme.
When power is on, an MS acquires a pilot channel. The MS performs an initial PN sequence phase search by satisfying a critical condition with respect to the starting points of a BS-generated PN sequence and an autonomously initiated PN sequence. If a call drop occurs, and PN reacquisition is required, the MS resumes the PN sequence phase search.
FIG. 1 illustrates an example of a PN sequence phase searcher using a serial search scheme in a CDMA mobile communication system.
Referring to FIG. 1, a controller 170 provides an overall control to the PN sequence phase searcher. Controller 170 also controls various parameters including an integration period, a window size, and an asynchronous accumulation period, and controls the phase transition of a PN sequence generated from a PN code generator 160. A signal input to the PN sequence phase searcher can be an RF-processed signal, i.e. down-converted, digitized, and applied through a modem chip at a mobile station. Here, the input signal includes a PN sequence generated by a specific BS.
A despreader 110 multiplies the signal received at a specific time point by the PN sequence received from PN code generator 160 to despread the signal. The starting point at which to search for the phase of the BS-generated PN sequence is preset. For example, the phase search starting point can be a PN offset ‘0’.
A synchronization accumulator 120 accumulates the output of despreader 110 for a corresponding integration period. An energy calculator 130 calculates a detection energy from the accumulated value according to the correlation between the BS-generated PN sequence and the MS-generated PN sequence. A comparator 140 compares energies each other and outputs max 4 energies and its PN phase. Controller 170 controls the phase transition of the PN sequence generated from PN code generator 160. If a reliable PN sequence phase, satisfying a predetermined condition, is acquired, an exterior controller 150 notifies an upper processor (not shown) of the acquired PN sequence phase. Then, the MS demodulates the signals received on a sync channel and a paging channel.
After the PN sequence phase search, controller 150 receives corresponding pilot offset information from the upper processor and controls various parameters of the receiver so that it can measure the reception strength of a pilot signal transmitted from an adjacent BS and compares it with the reception strength of a pilot signal in current service. This is called set management.
An MS manages information about the current BS at which the MS registers and other BSs. The MS receives an adjacent BS list message including the PN offset information of each BS on paging channels, measures the strength of a pilot signal received from each adjacent BS, and uses the measurement as a basis for determining a handoff. That is, the MS manages an active set, a neighbor set, and a candidate set. The active set corresponds to a BS in current communication the MS, the neighbor set includes BSs which are likely candidates for a handoff, and the candidate set includes BSs which are not in current use for data demodulation but have energy large enough for data demodulation.
FIG. 2 is a block diagram of another example of serial PN sequence phase searcher in a CDMA mobile communication system.
Referring to FIG. 2, multipliers 202 and 206 in a carrier demodulator 210 multiply an input signal by local carriers cos wct and sin wct, respectively, so that the input signal is demodulated to an in-phase signal I and a quadrature-phase signal Q. Matched filters 204 and 208 recover the waveforms of the signals I and Q, while a despreader 220 despreads the recovered signals I and Q by an I-arm PN code and a Q-arm PN code received from a PN code generator (not shown). Integrators 225 and 255 accumulate the despread signals I and Q for a predetermined integration period. Energy detectors 230 and 260 square the sums and calculate detection energies based on the correlation between a BS-generated PN code and an MS-generated PN code. An adder 235 adds the outputs of energy detectors 230 and 260. A comparator 240 compares the calculated detection energy with a threshold detection energy. A controller 250 feeds a corresponding PN phase control signal to the PN code generator according to the comparison result.
As described above, an MS can search for the PN sequence phase of a multi-carrier signal received from a BS using a serial search scheme or a parallel search scheme in a multi-carrier CDMA mobile communication system.
However, if the PN phases of different band signals are to be searched serially in the multi-carrier CDMA mobile communication system, a single serial PN sequence phase searcher should concurrently search a plurality of PN sequences (e.g., three PN sequences), thereby increasing time required for the PN sequence phase search and set management and remarkably decreasing reception performance. In particular, upon occurrence of a handoff, the single serial PN sequence phase searcher cannot rapidly cope with a channel change rate in a rapid channel changing environment. As a result, a call drop is more likely to occur.
This problem can be solved either by increasing the speed of a serial PN sequence phase searcher or by using a parallel PN sequence phase searcher. When serially searching for the PN sequence phases of different band signals in the multi-carrier CDMA mobile communication system, the single serial PN sequence phase searcher can operate at a rate N times higher (N is the number of the received band signals). For example, in a 3-band CDMA communication system, a 24xPN sequence phase searcher can be used instead of an 8xPN sequence phase searcher. The 24xPN sequence phase searcher can process a 3-band PN sequence but is difficult to achieve because of remarkably increased complexity in designing hardware.
A PN sequence phase searcher based on a parallel search scheme can be achieved by connecting serial PN sequence phase searchers as shown in FIG. 2 in parallel. Each PN sequence phase searcher searches for the phase of the PN sequence of a corresponding signal among multi-band input signals. If each PN sequence phase searcher performs a PN sequence phase search on a corresponding band input signal generated by a specific BS at the same search starting point, it implies that the same hypothesis at each band is subject to the PN sequence phase search. Therefore, the time required for the PN sequence phase search is almost equal to that in the serial search scheme. Since the performance of a PN sequence phase searcher is evaluated according to its capability of reducing an average search time, a method of reducing the average search time should be explored before applying the parallel search scheme to the multi-carrier CDMA mobile communication system.
Each BS has a unique pilot PN offset for identifying the BS in a CDMA mobile communication system. In the multi-carrier system, different carrier input signals or different band input signals transmitted by a specific BS have the same pilot PN offset. However, the band signals are not set in the same fading environment in view of the nature of a mobile communication environment. Therefore, the BS loads the same pilot signal on each band or carrier signal for transmission, so that an MS can search for the PN sequence phases of all band signals or all carrier signals.
Therefore, if each serial PN sequence phase searcher in the parallel search scheme performs a PN sequence phase search by generating a PN sequence at a different phase search starting point, the average of the PN sequence phase search time can be reduced. For example, to search a PN sequence with 32768 hypotheses, the 32768 hypotheses are divided into N, N being the number of the serial PN sequence searchers where each serial PN sequence searcher generates a PN code at the phase point of a corresponding segment. Thus, the time required for a PN sequence phase search can be reduced by N times.
Though the PN offset of a BS in the multi-carrier CDMA mobile communication system is the same in each band signal transmitted by the BS, a fading influence and a multi-path characteristic are different in each band. That is, there is no guarantee that each band signal received at an MS has the same PN sequence phase. Therefore, the multi-carrier CDMA mobile communication system should perform a PN sequence phase search on each of different band input signals.
When a specific serial PN sequence searcher completes a PN sequence phase search satisfying a predetermined condition, and a minimum PN sequence phase variation range is determined, other PN sequence phase searchers are searching for the PN sequence phases of their respective band input signals using different search conditions. The PN sequence phases of the other band input signals will be within the minimum phase variation period. Since each PN sequence phase searcher operates on a different search condition until the minimum phase variation period is determined, the average of PN sequence phase search times is reduced. Since the initial stage consumes most of the PN sequence phase search time, the PN sequence phase search time can be reduced by a factor of ½ or ⅓.
After the minimum PN sequence phase variation range is determined, each PN sequence phase searcher should implement a PN sequence phase search in the minimum phase variation period.